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interface-ip: overwrite resolv.conf file instead of renaming #62

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interface-ip: overwrite resolv.conf file instead of renaming #62

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63 changes: 52 additions & 11 deletions interface-ip.c
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Original file line number Diff line number Diff line change
Expand Up @@ -12,10 +12,12 @@
* MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
* GNU General Public License for more details.
*/
#include <errno.h>
#include <string.h>
#include <stdlib.h>
#include <stdio.h>
#include <libgen.h>
#include <sys/file.h>
#include <sys/stat.h>

#include <limits.h>
Expand Down Expand Up @@ -1578,6 +1580,22 @@ __interface_write_dns_entries(FILE *f, const char *jail)
void
interface_write_resolv_conf(const char *jail)
{
/*
* This implementation writes to a temporary file, calculates its CRC
* and compares it to the existing resolv.conf file CRC. If they differ,
* the existing resolv.conf is overwritten. This avoids unnecessary writes
* to resolv.conf which may trigger unnecessary reloads in dependent
* services. The existing resolv.conf is exclusively locked (flock)
* to prevent the possibility of concurrent writes (although this should
* not happen).
*
* The reason for not renaming the temporary file over the existing
* resolv.conf is that this causes issues with ujail environments where
* the jail has bind-mounted resolv.conf from the host. In this case,
* the new file would not be visible inside the jail until the jail was
* restarted.
*/

size_t plen = (jail ? strlen(jail) + 1 : 0 ) +
(strlen(resolv_conf) >= strlen(DEFAULT_RESOLV_CONF) ?
strlen(resolv_conf) : strlen(DEFAULT_RESOLV_CONF) ) + 1;
Expand All @@ -1596,34 +1614,57 @@ interface_write_resolv_conf(const char *jail)
strcpy(path, resolv_conf);
}

/* Write DNS entries to temporary resolv.conf file */
sprintf(tmppath, "%s.tmp", path);
unlink(tmppath);
f = fopen(tmppath, "w+");
if (!f) {
D(INTERFACE, "Failed to open %s for writing", path);
netifd_log_message(L_WARNING, "Failed to open temporary resolv.conf "
"(%s) for writing: %s", tmppath, strerror(errno));
return;
}

__interface_write_dns_entries(f, jail);

/* Calculate CRC of temporary file */
fflush(f);
rewind(f);
crcnew = crc32_file(f);
fclose(f);

crcold = crcnew + 1;
f = fopen(path, "r");
if (f) {
crcold = crc32_file(f);
fclose(f);
/* Open existing resolv.conf */
f = fopen(path, "w+");
if (!f) {
netifd_log_message(L_WARNING, "Failed to open resolv.conf (%s) "
"for writing: %s", path, strerror(errno));
return;
}

if (crcold == crcnew) {
unlink(tmppath);
} else if (rename(tmppath, path) < 0) {
D(INTERFACE, "Failed to replace %s", path);
unlink(tmppath);
/* Calculate CRC of existing resolv.conf */
crcold = crc32_file(f);

/* Overwrite resolv.conf if changed */
if (crcold != crcnew) {
/* Lock the file */
if (flock(fileno(f), LOCK_EX | LOCK_NB) < 0) {
netifd_log_message(L_WARNING, "Failed to exclusively lock "
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@systemcrash systemcrash Feb 18, 2026

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Failed to lock exclusively

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@dbnk0 dbnk0 Feb 18, 2026

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Do you want me to reword the message? It continues on the next line: Failed to exclusively lock resolv.conf (<PATH>): <ERROR MESSAGE>

Failed to lock exclusively resolv.conf (<PATH>): <ERROR MESSAGE> sounds a bit odd to me.

"resolv.conf (%s): %s", path, strerror(errno));
fclose(f);
return;
}

__interface_write_dns_entries(f, jail);

/* Unlock the file */
flock(fileno(f), LOCK_UN);

D(INTERFACE, "Updated resolv.conf (%s) content", path);
} else {
D(INTERFACE, "No change in resolv.conf (%s), no update needed", path);
}

fclose(f);
unlink(tmppath);
}

static void
Expand Down